Load compensated control system



July 23, 1940. H. E. HARTIG LOAD COMPENSATED CONTROL SYSTEM Filed Nov. 25, 1936 2 Sheets-Sheet l 3 m en WM July 23, 1940. H. E. HARTIG I 2,208,760

LOAD COMPENSATED CONTROL SYSTEM Filed NOV. 23, 1956 2 Sheets-Sheet 2 Patented July 23, 1940 2 UNITED STATES PATENT OFFICE LOAD COMPENSATEDCONTROL SYSTEM Application November 23, 1936, Serial No. 112,338

13 Claims.

This invention relates to automatic reset or load compensation mechanism in general and more particularly to that type of mechanism as applied to a follow-up control system.

It is an object of this invention to provide a follow-up control system of the electrical type utilizing a relay for positioning a device in accordance with changes in the value of a condition along with a reset or load compensation mechanism operative-as an incident to deviations of the condition to be controlled from the desired normal value for mechanically operating the relay to maintain the value of the condition to be controlled closer tothe desired normal value.

Another object of this invention is to provide a novel adjusting means for a reset mechanism whereby the rate of reset may be adjusted at will.

Other objects and advantages will become apparent to those skilled in the art upon reference to the accompanying specification, claims an drawings.

For a more thorough understanding of this invention, reference is made to the accompanying drawings in which:

Figure 1 diagrammatically shows an electrical type of follow-up system with one form of reset mechanism applied thereto;

Figures 2 and 3 show modified forms of the reset mechanism which may be utilized in the follow-up system of Figure 1;

Figure 4 is a partial diagrammatic view showing a further modified form of the reset mechanism in combination with the follow-up control system;

Figure 5 shows still another form of reset mechanism that may be utilized in the control system of Figure 1, and

Figure 6 shows the manner in which the reset mechanisms of Figures 1 to 5 may be adjusted to vary the rate of reset.

Referring now to Figure 1, a device to be positioned in a plurality of positions for controlling the value of a condition is generally designated at I0. Control means the state of which is varied in accordance with changes in the value of the condition is generally indicated at II. The control means II is adapted to operate a relay generally designated at I2. The relay I2 controls the operation of a motor generally designated at I3. The motor I3 positions the device III and trol system. An automatic reset or load compensation mechanism is generally designated at I6 for mechanically adjusting the relay I2 to maintain the condition to be controlled within narrower limits.

Although the control system of this invention may be utilized for controlling the value of any condition, it is disclosed for purposes of illustration as controlling a temperature condition and more particularly the temperature of a space (not shown). Therefore, the device III which is positioned in a plurality of positions is shown to be a valve I6 for controlling the supply of heating fluid to the space. The valve I6 may be operated by a valve stem I! connected by a pitman I8 to a crank disc I9 carried by a shaft 20 of the motor I3. The shaft 20 is operated through a reduction gear train 2I by motor rotors 22 and 23. The rotors 22 and 23 are operated by field windings 24 and 25, the arrangement being such that when field winding 25 is energized, the valve I6 is moved toward an open position and when the field winding 24 is energized, the valve I6 is moved toward a closed position.

The shaft 20 operates an abutment member 26 preferably made of insulating material for opening limit switches 27 and 28 when the valve I6 is moved to either an extreme open position or an extreme closed position, respectively. The shaft 20 also operates a slider 3|! with respect to a potentiometer resistance element 3I. The slider 36 and the resistance element 3| form a balancing potentiometer, the operation of which will be pointed out more fully hereafter. When the valve I6 is moved toward an open position, the slider 30 is moved to the left and when the valve I6 is moved toward a closed position, the slider 30 is moved to the right.

The control means generally designated at II is shown to be a thermostatic control means having a thermostatic element 33 responsive to variations in space temperature. Thermostatic ele- -ment 33 operates a slider 34 with respect to a potentiometer resistance element 35. The slider 34 and the potentiometer resistance element 35 form a control potentiometer. Upon a decrease in space temperature, the slider 34 is moved to the right in the direction indicated by the character C and upon an increase in space temperature, the slider 34 is'moved to the left in the direction indicated by the character H.

The relay generally designated at I2 may comprise an armature 31 pivoted at 38. The armature 31 carries cores 39 and 40 which are operated by relay coils 4| and 52. The armature 57 carries a switch arm 33 to be moved into engagement with spaced contacts 56 and 55. When the relay coil 4| is energized more than the relay coil 32, the switch arm 53 is moved into engagement with the contact 55 and when the relay coil 42 is energized more than the relay coil til, the switch arm 65 is moved into engagement with the contact as. When the relay coils M and c2 are equally energized, the switch arm (it is maintained spaced midway between the contacts 55 and 55 as shown in Figure 1.

The armature 5'! carries a lug 35 having an aperture therein through which extends a spring arm 51 carried by a member 55- The member 48 is, in turn, carried by and operated by bimetallic elements 69 and 5| which are suitably secured to stationary supports 55 and 52, respectively. The bimetallic elements 439 and 5| are heated by heaters 53 and 55. When the heaters 53 and 55 are equally energized, the member 58 and the spring arm i! are in a mid position as shown in the drawings and have no effect whatsoever on the armature 37. When the heater 55 becomes more highly energized than the heater 53, the bimetallic element 5| flexes downwardly in the direction indicated by the character H to move the member 58 and the spring arm iii in a clockwise direction to force the switch arm 63 in engagement with the contact 55. When the heater 53 becomes more highly energized than the heater 56, the thermostatic element 59 flexes downwardly in the direction indicated by the character H and rotates the member 58 and the spring arm- 41 in a counterclockwise direction. This resiliently'urges the switch arm 33 into engagement with the contact 46. Since a time interval exists between the time the energizations of the heaters 53 and 5d are changed and the time this change is felt by the thermostatic elements 49 and 5|, the switch arm 33 will not immediately be forced into engagement with the contacts 44 and 45.

Power is supplied to the relay I2 by means of a step-down transformer 55 having a primary 51 connected across line wires 58 and 59 and a secondary 60. One end of the secondary 60 is connected by a wire 5| to one end of the relay coil 4| and the other end of the secondary 60 is connected by a wire 52 to one end of the relay coil 42. The same end of the relay coil 4| is connected by a protective resistance 63 and a wire 64 to one end of the heater 53 and the other end of the heater 53 is connected by a wire 65 to the left end of the control potentiometer resistance element 35. The same end of the relay coil 42 is connected by a protective resistance 65 and a wire 61 to one end of the heater 54 and the other end of the heater 54 is connected by a wire 68 to the right end of the control potentiometer resistance element 35. The protective resistances 63 and 65 are also connected by wires 69 and 10 to the left and right ends, respectively, of the balancing potentiometer resistance element 3|. The other ends of the relay coils 4| and 42 are connected together by wires II and 12 and are connected by wires I3, I4 and 15 to the slider 34 of the control potentiometer and the slider 30 of the balancing potentiometer. By reason of these wiring connections, it is seen that the controlpotentiometer and the balancing potentiometer are connected in parallel with the series connected relay coils 4| and 42, and that the control potentiometer, the balancing potentiometer and the series connected relay coils 4| and 42 are connected across the secondary 50. It is also seen that the heaters 53 and 54 are connected in series with the left and right ends of the control potentiometer resistance element 35, respectively.

Omitting for the time being the operation 01 the heaters 53 and 55 of the reset mechanism l5, it is assumed that the parts are in the position shown in Figure 1. That .is, the space temperature is at the desired normal value and that the valve i5 is in a mid position for supplying just the correct amount of heat to the space to make up for the heat losses from the space. Upon an increase in the heating load, the space temperature decreases to move the slider 36 of the control potentiometer to the right in the direction indicated by the character C. By reason of the parallel relationship pointed out above, this right-hand movement of the slider 35 increases the current fiow through the relay coil 5| and decreases the current fiow through the relay coil 52 to cause the relay coil 5| to become more highly energized than the relay coil 52. As a result of these unequal energlzations of the'relay coils 5| and 42, the switch arm 52 is moved into engagement with the contact 55 to complete a circuit from the line wire 58 through wire 85, switch arm 33, contact 55, wire 8|, limit switch 21, wire 82, field winding 25, and wire 33 back to the other line wire 59. Completion of this circuit energizes the field winding 25 to move the valve 5 towards an open position to increase the supply of heat to the space. 0peration of the motor l3 to move the valve I5 towards an open position also causes left-hand movement of the slider 30 of the balancing DO- tentiometer. This left-hand movement of the slider flll increases the current fiow through the relay coil 62 and decreases the current flow through the relay coil 5|. When the slider 30 has moved sumciently far to the left to rebalance the energization of the relay coils 5| and 42, the switch arm 53 is moved out of engagement with the contact 55 to break the circuit through the field winding 25. In this manner, the valve I5 is modulated toward an open position in direct accordance with the amount of decrease in space temperature.

Upon a decrease in the heating load, the space temperature increases and the slider 34 is moved toward the left in the direction indicated by the character H. This left-hand movement of the slider 35 increases the current flow through the relay coil 42 and decreases the current flow through the relay coil 4| to increase the energization of the relay coil 42 and decrease the energization of the relay coil 4|. As a result of these unequal energizations of the relay coils 42 and 4|, the switch arm 43 is moved into engagement with the contact 44 to complete a circuit from the line wire 58 through wire 80, switch arm 43, contact 44, wire 84, limit switch 28, wire 85, field winding 24, and wire 83 back to the other line wire 59. Completion of this circuit energizes the field winding 24 to move the valve l6 toward a closed position which decreases the supply of heat to the space. Operation of the motor |3 to move the valve 6 towards a closed position causes right-hand movement of the slider 30 of the balancing potentiometer. This right-hand movement of the slider 3|! increases the current flow through the relay coil 4| and decreases the current fiow through the relay coil 42 and when the slider 30 has moved suficiently far to the right to rebalance the energization of the relay coils 4| and 42, the switch arm 43 is moved out of engagement with contact 44 to break the circuit through .the field winding 24.

In this manner, the valve I6 is modulated toward a closed position in direct accordance with the amount of increase in space temperature.

'By reason of the above construction, a true follow-up system of the electrical type is provided. It is found that if the control range of the control potentiometer is made sufliciently narrow to give an accurate temperature control action, hunting is likelyto occur. Also, it is found that if the control range of the control potentiometer is made sufficiently wide to eliminate hunting a droop in the space temperature occurs. That is, the space temperature is maintained at a lower value when the heating load is relatively great than when the heating load is relatively light. In order to eliminate hunting and to minimize this drooping" characteristic, the automatic reset mechanism generally designated at I5 is utilized and the operation of this reset mechanism it taken in connection with the follow-up control system is as follows:

Upon an increase in the heating load, the space temperature decreases and the slider M is moved to the right in the direction indicated by the character 0. The valve I6 is moved toward an open position in accordance with the amount of decrease in space temperature in the manner pointed out above. This movement of the'slider 3t to the right increases the current flow through the heater 5t and decreases the current flow through the heater 53 and after the lapse of a time interval, the thermostatic element flexes downwardly in the direction indicated by the arrow H and the thermostatic element (l9 flexes upwardly. This moves the spring arm ll] to the left to urge resiliently the switch arm it into engagement with the contact '35 which moves the valve it further towards an open position. Move= ment of the valve it further towards an open position causes left-hand movement or the slider to increase the energization oi the relay coil iii and decrease the energization of the relay coil t l. When the slider 3t has moved sumciently far to the left to overcome the resilient biasing action oi the spring arm ill, the switch arm (it is moved out of engagement with the contact 415 and further opening movement of the valve it is prevented. In this manner, the valve it is modu lated toward an open position in accordance with the amount or increase in space temperature and is moved further towards an open position by the action or the reset mechanism 65 to in= crease the supply of heat to the space to malre up for the increased heating load.

Upon a decrease in the heating load, the space temperature is increased and the slider lid is moved to the left in the direction indicated by the character H. The valve it is modulated toward a closed position in the manner pointed out above in direct accordance with the amount of increase in space temperature. This lefthand movement of the slider 34 alsoincreases the current flow through the heater 53 and decreases the current flow through the heater 5t and after the lapse of a time interval, the thermostatic element 49 flexes downwardly in the direction indicated by the arrow H and the thermostatic element 5| flexes upwardly to cause movement of the spring arm 41 to the right which, in turn, resiliently urges the switch arm 43 into engagement with the contact 44. The field winding 24 is thereupon energized and the valve I6 is moved further towards a closed position to decrease the supply of heat to the space to compensate for the decrease in heating load. Movement of the valve it towards a closed position causes right-hand movement of the slider 30 of the balancing potentiometer to increase the current fiow through the relay coil ll and decrease the current flow through the relay coil 42. When the slider 30 has moved sufiiciently far to the right to overcome the resilient biasing action of the spring arm 41, the switch arm 43 is moved out of engagement with the contact 44 and the circuit through the field winding 24 is broken. In this manner, the valve I6 is moved toward a closed position in accordance with the amount of increase in space temperature and is moved further toward a closed position in accordance with the amount of decrease in heating load.

By reason of this construction, the control range of the control potentiometer may be made sufilciently wide to prevent hunting and yet the drooping characteristic of the control system is reduced to a minimum whereby the space temperature is maintained within closer limits regardless of changes in load. If the time interval between the time of change in energizations of the heaters 53 and M and the time their efi'ect is sensed by the thermostatic elements it and iii is properly selected or adjusted, the space temperature will be maintained at a substantially constant value.

Referring now to Figure 2, I have shown an other manner for mechanically adjusting the relay 02 to compensate for changes in load. Like elements in Figures 1 and 2 have been designated by like reference characters. In Figure 2, the lug (it carried by the armature M is engaged by a spring arm til which is connected to one end of a thermostatic element ill. The other end of the thermostatic element ill is connected to one end of a thermostatic element 92 and the other end of the thermostatic element is connected to a suitable stationary support 93. When the thermostatic element 92 is heated up, it riexes in the direction indicated by the arrow H and when the thermostatic element 9t is heated up, it flexes in the direction indicated by the arrow H. The thermostatic elements Qi and 92 are heated by heaters 53 and 5G in exactly the same manner as in Figure 1. When the space temperature decreases due to an increase in the heating load, the heater 56 becomes more highly energized than the heater and as a result, the spring arm Jail is moved to'the left to urge Er.- siliently the switch arm 63 in engagement with the contact d5. When the space temperature increases as the result of a decrease in the heating load, the heater 53 becomes more highly energized than the heater it and the spring arm 963 is moved to the right to urge resiliently the switch arm 63 into engagement with the contact td. In this manner, the construction oi Figure 2 accomplishes exactly the same results as that of Figure l. and, therefore, a further description is not considered necessary.

Figure 3 shows a modified form of reset mechanism for accomplishing the same results as are accomplished in Figures 1 and 2. Like reference characters have been used for like elements in Figures 1 and 3. In Figure 3, the lug It carried by the armature 3? is provided with an enlarged aperture or a plurality of apertures to receive springs arms 95 and 96. These spring arms 95 and 86 are carried by thermostatic ele- Qli ments 9! and 98 which are suitably secured to a stationarysupport 99. Thermostatic element 91 is heated by the heater 53 and the thermostatic element 98 is heated by the heater 54. As the thermostatic elements 81 and 98 are heated, they flex inwardly in the direction indicated by the arrows H. Upon an increase in heating load which causes a decrease in space temperature, the heater 5% becomes more highly energized than the heater 53 and the spring arms 95 and 96 are moved to the left to urge resiliently the switch arm 53 into engagement with the contact 85. Upon an )ncrease in space temperature caused by a decrease in the heating load, the spring arms 95 and 96 are moved to the right to urge resiliently the switch arm 63 into engagement with the contact 456. In this manner, the reset mechanism of Figure 3 operates in exactly the same manner as the reset mechanism of Figure 1 and, therefore a further description is not considered necessary.

Figure 4 illustrates a modified form of reset mechanism wherein' a slightly more accurate control is obtained than could be obtained by the I construction shown in Figures 1, 2 and 3. Like elements of Figures 1 and 4 have been designated by like reference characters. In Figures 4, the relay coil .di is connected by wires 53' and lfll, heater I92, wire I03, resistance ltd, and wire N35 to the left end of the balancing potentiometer resistance element 3!. The relay coil 62 is connected by wires t6 and it, heater ifi'i, wire I08, resistance W9, and wire Mil to the right end of the balancing potentiometer resistance element The heaters "32 and it? also heat the bimetallic elements 69 and 54. When the valve it is moved toward an open position and the slider 30 ismoved to the left upon an increase in the heating loadas pointed out in connection with Figure 1, the current flow through the heater m2 is increased and the current flow through the heater it"! is decreased to urge further the switch arm 39 into engagement with the contact 65. As a result of the increased energization of the heater I02 and the decreased energization of the heater ml, the valve It is moved slightly further towards an open position to further increase the supply of heat to the space. In a like manner, valve it is moved toward a closed position and th'esilder 3b is moved toward the right as the result of a decrease. in the heating load. The energization of the heater ml is increased and the energization of the heater m2 is decreased to urge further the switch arm 63 into engagement with the contact 66. The heaters I02 and mi, therefore, act to move the valve It further towards an open or towards a closed position in accordance with the increase or decrease in the heating load whereby the droop of the control system in further decreased and the space temperature is maintained within narrower limits. Therefore, a slightly more accurate control is obtained by the means shown in Figure 4 than can be obtained by the means shown in Figures 1, 2 and 3. Even with the closer control afforded by Figure 4, hunting is substantially prevented. The resistances not and I09 are provided to decrease the heating effect of the heaters I02 and I0? to prevent hunting of the system. Since the heaters i8? and it2'are in series with the balancing potentiometer resistance element 3| and since the heaters 53 and 54 are in series with the control potentiometer resistance element 35, the protective resistances 69 and 66 of Figures-1, 2 and 3 are not needed. In other aaospec words, the heaters I02 and m and the heaters 59 and 54 protect the control system from being short-circuited.

Figure 5 shows a different manner for accomplishing substantially the same results as are accomplished in Figures 1, 2 and 3. In Figure 5, the protective resistances 63 and 66 are connected directly by wires 64 and 67 to the left and right ends, respectively, of the control potentiometer resistance element 35. The armature 311 instead of being provided with a. lug 55 it is provided with a pin H2 located below the pivot point 38. One end of spring H3 is connected to the pin H2 and the other end of the spring H3 is connected to a bracket lid carried by a thermostatic element H5. Thermostatic element H5 is carried by an ambient temperature com-,- pensating thermostatic element H6 which is, in turn, suitably secured to a stationary support iii. A heater 8 is adapted to heat the thermostatic element H5. The junction of the relay coils M and 92 is connected by wires '13 and H9 to one end of the heater H8 and the other end of the heater H8 is connected by a wire I20 to the slider 341 of the control potentiometer. The control potentiometer, the balancing potentiometer and the series connected relay coils M and 62 are all connected in parallel in the manner pointed out in connection with Figure 1. The connection between the junction of the relay coils 3! and Q2 and the slider 36 of the control potentiometer includes the heater H8. When the slider 36 of the control potentiometer is in the mid position, no current passes through the heater H8. The spring H3 is so arranged that when the current fiow through heater H8 is zero, it is in a neutral condition and. has no efiect whatsoever upon the armature 3L Upon an increase in the heating load, the

space temperature decreases and the slider 36 is moved toward the right. By reason of the above parallel relationship, current then flows through heater M8 to cause heating of the thermostatic element M5 to flex the outer endthereof upwardly in the direction indicated by the arrow H. This places the spring H3 in tension and since it is connected to the armature 31 below the pivot point 38, the spring H3 resiliently urges the switch arm 63 into engagement with the contact 435. As long as the slider 36 of the control potentiometer is to the right of the mid position shown, the heater H8 will be energized and the switch arm 83 will be resiliently urged in engagement with the contact 35. By reason of this construction, the valve i6 is modulated toward an open position and is moved further A toward an open position in accordance with the decrease in space temperature and the increase in heating load in the same general manner as pointed out in connection with Figure 1.

Upon a decrease in the heating load which causes an increase in space temperature, the slider 36 is moved to the left in the direction indicated by the character H to cause current flow through heater "8 to again flex upwardly the thermostatic element H5 to place the spring H3 in tension. Spring H3 therefore urges the switch arm (33 into engagement with the contact M to cause further closing movement of the valve it. Therefore, the valve i6 is modulated toward a closed position in accordance with the increase in space temperature and is moved-further towards the closed position in accordance with the decrease in heating load. The structure of Figure 5. therefore, accomplishes substantially the same results as are accomplished in Figures 1 to 3.

Since in Figures 1 to 4 the current flow through the heaters 53 and 54- is proportional to the amount of deviation in space temperature and since in Figure 5 the current flow through the heater 8 is proportional to the amount of-deviation in space temperature, the amount that the valve is is repositioned or reset towards an open or closed position is dependent upon the amount of deviation in space temperature. It follows also that the rate at which the valve I6 is so repositioned or reset is proportional to the amount of deviation in space temperature.

In order to adjust the rate of reset and consequently the amount of reset of the valve l6, provision must be made for varying the time delay or time lag between the change in energize.- tion of the heaters 53, 54 and H8 and the time that these changes in energizations are felt by the thermostatic elements 49, 5| and H5. It the time delay is relatively long, the amount and rate of reset is less than if the time delay is relatively short. Figure 6 shows one manner in which this control of the time delay may be accomplished. For purposes of illustration, the thermostatic elements oi Figures 1 to 5 are shown to be a bimetallic element 2 in Figure 6. The bimetallic element E22 is suitably secured to a stationary support I23. Secured to the bimetallic element H2 in any suitable manner, as by rivets M5, is a heat conducting member tit and mounted on the heat conducting member 02d is a heater B26 which may correspond to the various heaters of Figures 1 to 5. The heat generated by the heater 825 is conducted through the member lid to the bimetallic element 822. Therefore, the member 92 3 forms a heat conducting path. The time delay between the change in energization of heater 8% and the time in which the thermostatic element (I22 senses this change is dependent upon the rate at which heat is conducted through the member Hid. In order to change this rate of heat conduction for this time at which the thermostatic element i122 senses a change in the energization of the heater members lid of different materials may be substituted for the one shown. in other words, members of difierent heat conducting characteristics may be substituted for the member H2 3 to accomplish the desired results. Also, as shown in Figure 6, the heater Q26 may be slidably mounted on the member 026 and if the heater 0% is moved to a position relatively close to the thermostatic element G222, a change in the energizatinn thereof will be sensed more rapidly by the thermostatic element i222 than if the heater i125 were moved to a position more remote from the thermostatic element 922. Therefore, by reason of the construction of Figure 6 provision is made for adjusting the rate and amount of reset of the valve it and the structure of Figure 6 may be inserted in any of the Figures 1 to for accomplishing the desired results.

Although for purposes of illustration I have shown several forms of-this invention, itis obvious that other forms thereof may become obvious to those skilled in the art upon reference to this specification and, therefore, my invention is to be limited only by the scope of the appended claims and prior art.

I claim as my invention:

1. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, a control potentiometer, means for adjusting the control potentiometer in accordance with changes in the value of the condition to be controlled, said control potentiometer having a desired adjustment according to a desired normal value of the condition, a normally balanced relay for controlling said device, a balancing potentiometer adjusted by said device, connections between the relay, the control potentiometer and the balancing potentiometer for positioning said device in accordance with changes in the value of the condition to be controlled, resilient means mechanically connected to the relay for also controlling the operation thereof, thermostatic means for adjusting said resilient means, and electric heating means for the thermostatic means controlled by the current flow through said connections to position said device difierently with respect to the value of the condition to be controlled.

2. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, a control potentiometer including a resistance element and a slider element, means for moving the elements with respect to each other in accordance with'changes in the value of the condition to be controlled, a relay including an armature and a pair of series connected relay coils for controlling the operation of the device, first connections between the ends of the resistance element and one end of each relay coil, a second connections between the slider element and the junction of the relay coils, a balancing potentiometer including a resistance element and a slider element, means operated by said device for moving the elements of the balancing potentiometer with respect to each other, third connections between the ends of the balancing potentiometer and the first mentioned ends of the relay coils, fourth connections between the balancing potentiometer slider element and the junction of the relay coils whereby the device is positioned in accordance with variation in the value of the condition to be controlled, resilient means for mechanically adjusting the armature of the relay, and means responsive to the current flow through the first connections for adjusting the resilient means to position said device differently with respect to the value of the condition to be controlled.

3. in combination, a device to be positioned in a plurality of positions for controlling the value of a condition, a control potentiometer including a resistance element and a slider element, means for moving the elements with respect to each other in accordance with changes in the value of the condition to be controlled, a relay I including an armature and a pair oi series connected relay coils for controlling the operation of the device, first connections between the ends of the resistance element and one end of each relay coil, second connections between the slider element and the junction of the relay coils, a balancing potentiometer including a resistance element and a slider element, means operated by said device for moving the elements of the balancing potentiometer with respect to each other, third connections between the ends of the balancing potentiometer and the first mentioned ends of the relay coils, fourth connections between the balancing potentiometer slider element and the junction of the relay coils whereby the device is positioned in accordance with variation in the value of the condition to be controlled, resilient means for mechanically 7 dill adjusting the armature of the relay, and means responsive to the current flow through the second connections for adjusting the resilient means to position said device differently with respect to the value of the condition to be controlled.

4. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, a control potentiometer including a resistanc element and a slider element, means for moving the elements with respect to each other in accordance with changes in the value of the condition to be controlled, a relay including an armature and a pair of series connected relay coils for controlling the operation of the device, first connections between the ends of the resistanceelement and one end of. each relay coil, second connections between the slider element and the junction of the relay coils, a balancing potentiometer including a resistance element and a slider element, means operated by said device for moving the elements of the balancing potentiometer with respect to each other, third connections between the ends of the balancing potentiometer and the first mentioned ends of the relay coils, fourth connections between the balancing potentiometer slider element and the junction of the relay coils whereby the device is positioned in accordance with variation in the value of the condition to be controlled, resilient means for mechanically adjusting the armature of. the relay, and means responsive to the current flow through the first and third connections for adjusting the resilient means to position said device diiierently with respect to the value of the condition to be controlled.

5. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, adjustable control impedance means adjusted in accordance with changes in the value of the condition to be controlled, follow-up means operated as an incident to operation of the device, relay means in control of said device and operated by said control impedance means and said follow-up means for positioning said device in accordance with the adjustment of said control impedance means for maintaining the condition within certain limits, resilient means mechanically connected to said relay means for also controlling the operation thereof, and means for adjusting said resilient means for positioning said device differently with respect to the value of the condition to be controlled.

6. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, adjustable control impedance means adjusted in accordance with changes in the value of the condition to be controlled, follow-up means operated as an incident to operation of the device, relay means in control of said device and operated by said control impedance means and said follow-up means for positioning said device in accordance with the adjustment of said control impedance means for maintaining the condition within certain limits, resilient means mechanically connected to said relay means for also controlling the operation thereof, and means operative as an incident to deviation of the value of the condition from the desired normal value for adjusting said resilient means additionally to position the device with respect to the adjustment of the control impedance means for returning the value of the condition toward the desired normal value.

7. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, adjustable control impedance means adjusted in accordance with changes in i the value of the condition to be controlled, ad-

justable balancing impedance means adjusted as an incident to operation of the device, relay means in control of said device for controlling the direction and extent of operation of the device, connections between the control impedance means, the balancing impedance means and the relay means for controlling the relay means in accordance with the adjustments of. the control and balancing impedance means to position the device in accordance with the adjustment of the control impedance means to maintain the condition within certain limits, resilient means mechanically connected to the relay means for also controlling the operation thereof, and means operative as an incident to deviation of the value of the condition from the desired normal value for adjusting said resilient means additionally to position the device with respect to the adjustment of the control impedance means for returning the value of. the condition toward the desired normal value.

' 8. In combination, a device to'be positioned in a plurality of positions for controlling the value of a condition, adjustable control impedance means adjusted in accordance with changes in the value of the condition to be controlled, adjustable balancing impedance means adjusted as an incident to operation of the device, relay means in control of said device for controlling the direction and extent of operation of the device, connections between the control impedance means, the balancing impedance means and the relay means for controlling the relay means in accordance with the adjustment of the control and balancing impedance means to position the device in accordance with the adjustment of the control impedance means to maintain the condition within certain limits, resilient means mechanically connected to the relay means for also controlling the operation thereof, and means responsive to the current flow in said connections for adjusting said resilient means additionally to position the device with respect to the adjustment of the control impedance means for returning the value of the condition toward the desired normal value.

9. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, adjustable control impedance means adjusted in accordance with changes in the value of the condition to be controlled, adjustable balancing impedance means adjusted as an incident to operation of the device, relay means in control of said device for controlling the direction and extent of operation of the device. connections between the control impedance means, the balancing impedance means and the relay means for controlling the relay means in accordance with the adjustment of the control and balancing impedance means to position the device in accordance with the adjustment of the control impedance means to maintain the condition within certain limits, resilient means mechanically connected to the relay means for also controlling "the operation thereof, and thermoelectric means responsive to the current flow in said connections for adjusting said resilient means additionally to position the device with respect to the adjustment of the control impedance means for returning the value of the condition toward the desired normal value.

10. In. combination, a device to be positioned in a plurality of positions for controlling the value of a condition, adjustable control impedance means adjusted in accordance with changes in the value of the condition to be controlled, adjustable balancing impedance means adjusted as an incident to operation of the device, relay means in control of said device for controlling the direction and extent of operation of the device, connections between the control impedance means, the balancing impedance means and the relay means for controlling the relay means in accordance with the adjustment of the control and balancing impedance means to position the device in accordance with the adjustment of the control impedance means to maintain the condition within certain limits, resilient means mechanically connected to the relay means for also controlling the operation thereof, thermostatic means for adjusting the resilient means, and heating means responsive to the current flow in said connections for heating the thermostatic means additionally to position the device with respect to the adjustment of the control impedance means for returning the value of the condition toward the desired normal value.

11. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, adjustable control impedance means adjusted in accordance with changes in the value of the condition to be controlled, adjustable balancing impedance means adjusted as an incident to operation of the device, relay means in control of said device for controlling the direction and extent of operation of the device, first connections between the control impedance means and the relay means and second connections between the balancing impedance means and the relay means for controlling the relay means in accordance with the adjustment of the control and balancing impedance means to position the device in accordance with the adjustment of the control impedance means to maintain the condition within certain limits, resilient means mechanically connected to the relay means for also controlling the operation thereof, and means responsive to the current fiow in the first connections for adjusting said resilient means additionally to position the device with respect to the adjustment of the control impedance means for returning the value of the condition toward the desired normal value.

12. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, adjustable control impedance means adjusted in accordance with changes in the value of the condition to be controlled, adjustable balancing impedance means adjusted as an incident to operation of the device, relay means in control of said device for controlling the direction and extent of operation of the device, first connections between the control impedance means and the relay means and second connections between the balancing impedance means and the relay means for controlling the relay means in accordance with the adjustment of the control and balancing impedance means to position the device in accordance with the adjustment of the control impedance means to maintain the condition within certain limits, resilient means mechanically connected to the relay means for also controlling the operation thereof, and means responsive to the current flow in the first and second connections for adjusting said resilient means additionally to po sition the device with respect to the adjustment of the control impedance means for returning the value of the condition toward the desired normal value.

13. In combination, a device to be positioned in a plurality of positions for controlling the value of a condition, adjustable control impedance means adjusted in accordance with changes in the value of the condition to be controlled, adjustable balancing impedance means adjusted as an incident to operation of the device, relay means in control of said device for controlling the direction and extent of operation of the device, connections between the control impedance means, the balancing impedance means and the relay means for controlling the relay means in accordance with the adjustment of the control and balancing impedance means to position the device in accordance with the adjustment of the control impedance means to maintain the condition within certain limits, resilient means mechanically connected to the relay means for also controlling the operation thereof, thermostatic means for adjusting the resilient means, heating means responsive to the current flow in said connections for heating the thermostatic means additionally to position the device with respect to the adjustment of the control impedance means for returning the value of the condition toward the desired normal value, and means for adjusting the heating effect of the heating means.

HENRY E. HARTIG. 

